In the course of preparing a cellulose pulp to be used for further processing, such as for example in papermaking or the like, a number of “standard” operations are often performed. For chemical pulp, lignin from wood chips is dissolved in a digester in order to separate the fibers within the wood chips from each other. The cooked pulp is then transported to washing and screening devices. During cooking, not all of the wood chips are equally well digested, and the resulting pulp thus contains not only individually separated fibers, but also pieces of uncooked wood chips, knots and fiber bundles known as shives. The shives, knots and other impurities (e.g. sand, bark, etc.) still remaining in the pulp may cause problems in later stages of the pulp processing, and thus need to be removed. There are a number of well-known operations used, separately or in combination, to separate impurities from the pulp, including sedimentation, screening and vortex cleaning operations.
Screening refers to an operation in which fibers in a pulp suspension pass through a perforated plate with holes or slots, while the impurities are retained. The fraction of pulp passing the plate is referred to as the accept fraction or simply the accept. The fraction of pulp not passing through the openings of the plate is referred to as the reject fraction or simply the reject. The pulp fraction fed to the perforated plate is referred to as the inject fraction or simply the inject. An inject can thus be said to be a pulp flow about to be divided into an accept and a reject. Instead of a perforated plate, slots for separation may be created in other ways, e.g. through forming a screen basket of longitudinal bars.
In a pulp mill, there are often several screening operations at different locations throughout the process. For example, there is often a primary screening operation, in connection with the pulp being washed and dewatered, and an after-screening operation after bleaching of the pulp. Each screening operation can, and often will be, performed in several stages. One single screening apparatus is often not sufficient to separate all impurities in one stage. In order to achieve good screening efficiency, the reject flow has to be sufficiently large to make sure that the impurities not passing through the screen are removed from the screening apparatus. In case the reject flow is too small, impurities may be retained in the screening chamber where they can cause unnecessary wear to the screen. Since the reject flow must therefore be of a certain magnitude, this implies that a large number of “good” fibers (i.e. fibers that would belong to the accept) is not given enough time to pass the perforated plate or through slots between longitudinal bars, but instead becomes a part of the reject. The accept portion from the screening stage is passed to the next processing step, while the reject portion is passed to a subsequent screening stage, in order to be screened again. In order to minimize fiber losses, the accept from the subsequent screening stage is then returned to the inject of the preceding screening stage. In that way, most of the “good” fibers are recovered.
There are several different kinds of screening apparatuses. One commonly used screening apparatus is a so-called combi-screen, meaning that two or more screening stages are combined within the same screen housing. An example of such an apparatus is disclosed in European Patent No. 1,165,882, where a first screening means is located at least partly within a second screening means. Combi-screens have been developed in order to provide a cheaper process in which two different screening apparatuses are combined into one apparatus, eliminating the need for e.g. a separate knotter. There are also other kinds of multistage screens in which several screening stages are combined within the same apparatus. For example, the separate screening stages may be arranged by means of screens on top of each other, or by one high screening basket being divided into separate screening sections
While using a combi-screen, it is not possible to return the accept from a subsequent screening stage after the combi-screen to its directly preceding screening stage, such as e.g. a fine screen contained within the combi-screen. The accept portion from the subsequent screening stage (at this stage not containing any larger particles), when returned to a combi-screen, will have to pass both screening stages within the combi-screen again, which implies an unnecessary load on the coarse screen. Part of the pulp entering the coarse screen has then already been screened for larger impurities, and does not require the coarse screening stage. The return flow from the subsequent screening stage can submit to about 15-20% of the main flow into the combi-screen. This implies that the pump before the combi-screen must be dimensioned to handle a larger flow, in case the return flow is to be added before the pump. Even if the flow is added after the pump, the coarse screen still has to be dimensioned to be able to handle the larger flow. All of these measures render a more expensive process.
One object of the present invention is thus to provide an improved system and arrangement for the screening of cellulose pulp. Another object of the present invention is to achieve a more efficient way of screening while minimizing fiber losses, and at the same time keeping the number of individual screening apparatuses as low as possible, in order to minimize the total investment cost. Another object of the present invention is to provide a screening system and an arrangement where the screening means and associated equipment does not have to be overdimensioned, also in order to minimize the investment cost.